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Re: multiply-connected graph of schema import

From: C. M. Sperberg-McQueen <cmsmcq@blackmesatech.com>
Date: Fri, 15 Apr 2011 09:52:21 -0600
Cc: "C. M. Sperberg-McQueen" <cmsmcq@blackmesatech.com>, Michael Kay <mike@saxonica.com>
Message-Id: <715EF42D-BD7D-4254-8444-BCA706AC4E29@blackmesatech.com>
To: xmlschema-dev@w3.org

On Apr 15, 2011, at 4:57 AM, Michael Kay wrote:

> On 14/04/2011 14:15, Paul Harrison wrote:
>> Is a multiply-connected (but non-circular) graph of schema imports allowed?
>> 
>> e.g  B imports A
>>        C imports B
>>        D imports C and A
>> 
>> Some tools seem to get a little confused by this sort of thing
>> 
>> Regards,
>> 	Paul
>> 
>> 
> It's not only allowed, it's often essential.
> 
> The rule is that if a schema document A references a schema component in namespace N, then it must include an xs:import for namespace N.
> 
> It's true however that the rules in the spec leave a lot to be desired (XSD 1.1 improves them, though not as much as I would like), and as a result, implementations are not 100% consistent.
> 
> xs:include is different - there's never a need to have circular includes, but they certainly shouldn't cause any problems if you do.



'Should' may be a key word in that last sentence; "shouldn't" cause
problems is not the same as "won't" cause problems.

The text of 1.0 defines inclusion in terms of 'the schema corresponding
to' the included schema document, and its effect on 'the schema
corresponding to' the including schema document.  So if A includes
B, we can know what schema corresponds to A only by first calculating 
the schema corresponding to B.  In the case of circular includes, the
schema corresponding to Be can be calculated only by first
calculating the schema corresponding to A.  But we do not yet
know what that is, because we have not yet calculated all the 
things it depends on.  We have infinite regress, caused by
a recursive definition that identifies no base case.

The most natural implementation of this definition is to turn the
circular case into an infinite loop, and that is in fact what some 
implementations have done.  Other implementations of XSD 1.0 
have taken the view that circular includes are legal and have a 
well defined meaning, but I do not know how they derive that 
conclusion from the text of the spec, and in fact I do not believe
they can.  When I have asked implementors about it, the answer 
has usually amounted to a shrug; it has never involved any appeal
at all to the actual words of the spec.

So while some 1.0 processors will process circular includes 
without complaint, schema authors interested in having their schema 
documents work with more than one 1.0 processor would be wise 
to avoid circular includes.

In 1.1, an explicit rule is added to justify stopping the recursion.
But I think that was a substantive not an editorial change.

-- 
****************************************************************
* C. M. Sperberg-McQueen, Black Mesa Technologies LLC
* http://www.blackmesatech.com 
* http://cmsmcq.com/mib                 
* http://balisage.net
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Received on Friday, 15 April 2011 15:52:49 GMT

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